Spoiler bbcode tweaks

 - rename [spoilers] to [spoiler] since that seems to be more standard.
   Hopefully no one is depending on this too much yet.
 - strip leading & trailing whitespace inside spoiler tags. I'm not sure
   whether this is standard, but I definitely think it is better.
Merge pull request #48 from eve-val/feature/spoiler-tags

Spoiler bbcode tweaks (fixes #45)